There is a strong commercial demand for propionic acid as a source for propionic esters that are used for producing natural fruit flavors, but its use is hampered by the high cost of producing it. It is well known that when S. ruminantium grows on a lactic acid or lactate medium, it metabolizes the medium to CO.sub.2 and a mixture of propionic, acetic, and succinic acids. See, for instance U.S. Pat. No. 4,138,498, which is concerned with the removal of excess lactic acid in the diet of ruminants by addition to the diet of a co culture containing S. ruminantium. Also an extensive article by Linehan, et al.,published in 1978, disclosed essentially experimental scale data relating to the title subject, "Nutritional Requirements of Selenomas ruminantium for Growth On Lactate, Glycerol, or Glucose"; see Applied and Environmental Microbiology, Vol. 35, No. 2. The Linehan et al. studies of comparative growth rates were not concerned with processing and related yield factors that would be relevant to the commercial production of propionic, acetic, and succinic acids that can be purified for use in flavors.
Co-culture processes that may use S. ruminantium are disclosed in U.S. Pat. Nos. 1,910,130 and 4,794,080, but are not suitable for making propionic acids for commercial use in flavors; they have serious disadvantages, including the difficulty of maintaining stability of the co culture and avoiding contamination by other microorganisms.
The commercial value of flavors is enhanced if they can be characterized as "natural" products, that is, for instance, if they are plant or animal products or if they are made from natural starting materials by biological fermentation or enzymatic alteration. When selling kosher foods, their value is further enhanced if they are not made from meat or a dairy product, and therefore are "parve", so that only one inventory of the flavor is required. Synthetic propionic acid flavors, which are not "natural", and those made from the lactose in whey by strains of Propionibacterium acidipropionici (as disclosed in many U.S. Patents, such as U.S. Pat. Nos. 1,459,959, 1,865,146, and 4,676,987), which produce propionic acid, are not appropriate for commercial production of natural kosher flavoring materials.
There is therefore a need for a process for preparing propionic acid salts by fermentation of a natural starting material, that can produce kosher propionates for conversion into propionate esters for use in flavoring foods after conventional processing and purification, and in commercially economic yields, for instance, 2 to 3% propionic acid (as sodium propionate), produced by fermentation for about 72 hours, and preferably for about 48 hours.